sip: fix sip call crash

MTU discovery was only implemented into TLS secured ICE transport,
not in other sip transport.
This wasn't taken in account and causes crashes when SIP call (TLS or not)
are made.
We fix the problem by detecting the transport type at various stage
and calling correct API (or use default values as in the case of MTU).

Change-Id: Id256a718ca8265a7295085fab8db9cf8e4c99683